Differential cytokine production by CD4+ T cells is an important mechanism for selectively regulating immune effector function. The expression f the interleukin-4 (IL-4) and interleukin-2 (IL-2) genes is differentially regulated during the maturation of antigenically-naive, virgin T cells into memory cells. Virgin cells produce IL-2 but low or undetectable amounts of IL-4, while memory T cells may produce high levels of both cytokines. The first aim of this project is to determine the ability of increased IL-4 production by activated T cells in vivo to influence the outcome of the immune response. For this aim, PIL-2/IL-4 transgenic mice have been generated in which the IL-2 promoter drives transcription of an IL-4 transgene. This dramatically increases IL-4 expression by T cells, presumably by virgin an memory cells, and increases serum IgE levels, a hallmark of IL-4 activity, in an activation-dependent fashion. It is predicted that the transgene will also: 1) increase the bias of CD4+ T cell cytokine production, in bulk and at the clonal level, towards a Th2 pattern (increased production of endogenous-gene derived IL-4 and IL-5, decreased production of IL-2 and interferon-gamma), 2) decrease cell- mediated immunity, including delayed-type hypersensitivity and 3) enhance expression of IgE but not other isotypes. To test these predictions, PIL- 2/IL-4 mice will be challenged with antigens and the effect of the transgene on modifying isotype expression by B cells, antigen-specific delayed-type hypersensitivity, and endogenous cytokine gene expression by CD4+ T cells will be determined. In subsequent experiments, PIL-2/IL-4 mice and littermate controls will be challenged with pathogens that normally result in either protective Th2-type (H. polygyrus) or Th1-type (Leishmania major) responses. We will determine whether the transgene will alter cytokine production in vivo, IgE expression, and the burden of infection. The second aim of this project is to identify cis-elements in the promoter of the IL-4 gene that confer increased IL-4 gene expression by memory cells and 2) to identify selective differences in potential transcription factors binding to these elements, that differ between virgin and memory T cells, using gel-shift assays and in vivo footprinting. The in vivo relevance of cis-regulatory elements defined by in vitro assays will then be tested by generating mice transgenic for human IL-4 genomic constructs. We will test whether the cis-elements of the transgene will confer, in a manner analogous to the endogenous IL-4 gene, enhanced expression by in vitro-primed T cells, memory T cells, and by T cells in vivo after helminth challenge. These studies will provide substantial insight into the biologic significance of the differential regulation of IL-4 during in vivo immune responses, as well as the molecular mechanisms responsible for this regulation. They may also point out means by which endogenous IL-4 production might be inhibited, such as in IgE-mediated allergic disorders, or, alternatively, increased, as in cytokine therapy for cancer. These studies may also point out potential pitfalls of these alterations.